Low-heat, mild alkaline pretreatment of switchgrass for anaerobic digestion

Abstract

This study examines the effectiveness of alkaline pretreatment under mild heat conditions (100°C or 212°F) on the anaerobic co-digestion of switchgrass. The effects of alkaline concentration, types of alkaline, heating time and rinsing were evaluated. In addition to batch studies, continuous-feed studies were performed in triplicate to identify potential digester operational problems caused by switchgrass co-digestion while accounting for uncertainty due to digester variability. Few studies have examined anaerobic digestion of switchgrass or the effects of mild heating to enhance alkaline pretreatment prior to biomass digestion. Results indicate that pretreatment can significantly enhance digestion of coarse-ground (≤ 0.78 cm particle size) switchgrass. Energy conversion efficiency as high as 63% was observed, and was comparable or superior to fine-grinding as a pretreatment method. The optimal NaOH concentration was found to be 5.5% (wt/wt alkaline/biomass) with a 91.7% moisture level. No evidence of operational problems such as solids build-up, poor mixing, or floating materials were observed. These results suggest the use of waste heat from a generator could reduce the concentration of alkaline required to adequately pretreat lignocellulosic feedstock prior to anaerobic digestion.

Keywords:

• Anaerobic digestion
• alkaline pretreatment
• biogas
• switchgrass
• NaOH

Acknowledgments

Bench-top digester seed culture and feed sludge was supplied by Bloomington/Normal Wastewater Reclamation District. We would also like to recognize our undergraduate research assistants: Doug Becker, Alex Choo, Melissa Gawron, Chris Lund, Morgan Martin, and Tony Schwegmann.